Medulloblastoma, the most common malignant pediatric brain tumor, can result from disruption of the delicate balance between proliferation and differentiation in cerebellar granule neuron precursors (CGNPs). After birth, CGNP proliferation is dependent on signaling by Sonic hedgehog (Shh), which rapidly expands the pool of progenitor cells in the cerebellar external granule layer (EGL). Aberrant activation of the Shh signaling pathway can lead to the formation of medulloblastomas by driving continued cycling in immature, proliferating CGNPs. However, the mechanistic link between Shh deregulation and medulloblastoma formation remains poorly understood. Preliminary microarray studies in the lab demonstrated that expression of Special AT-rich Binding Protein 1 (Satb1) is upregulated in our novel mouse model of hedgehog-dependent medulloblastoma. Satb1 is a nuclear protein that functions as a 'genome organizer' essential for proper T-cell development and breast cancer growth and metastasis; it regulates gene expression by recruiting chromatin remodeling or modifying enzymes and transcription factors to genomic DNA, which it tethers via specialized DNA sequences highly potentiated for unpairing. In breast cancer cells, once Satb1 is expressed, it coordinates expression of a large number of genes to induce metastasis. Removal of Satb1 from aggressive breast cancer cells reverses metastatic phenotypes and inhibits tumor growth, indicating its key role in breast cancer progression. However, its role in the brain during development and tumorigenesis has yet to be elucidated. Consistent with our preliminary microarray studies, we find that Satb1 mRNA is localized to proliferative CGNPs during development, and that Shh signaling positively regulates Satb1 protein levels in cultured CGNPs. Therefore, in the current proposal, we hypothesize that Satb1 is a novel effecter of the Shh signaling pathway in driving CGNP proliferation and medulloblastoma formation. To investigate this, we have two specific aims. First, we will determine the requirement for Satb1 in CGNP proliferation and cerebellar development in vitro and in vivo. We will modulate expression of Satb1 using a gain and loss-of-function approach in cultured primary CGNPs and using in vivo electroporation. In our second aim, we seek to define the role of Satb1 during hedgehog-mediated oncogenic transformation. We will perform Satb1 knockdown to determine effects on proliferation of hedgehog-dependent medulloblastoma cells, and assess the in vivo effects of Satb1 knockdown by allografting medulloblastoma cells expressing Satb1 shRNA into the cerebella of mice and monitoring tumor growth. A deeper understanding of the molecular basis of medulloblastoma is crucial for improving early diagnosis and treatment of this devastating disease.